One of the important processes for the application of coating materials to commercial products is the use of electrostatic powder coating techniques. Electrostatic coating of products with liquid coating materials is, of course, well known and highly developed. However, the need for using volatile solvents as vehicles in connection with most liquid coating materials presents increasingly difficult problems in terms of solvent recovery techniques and systems, to satisfy antipollution regulations and, more currently, in terms of the difficulty of obtaining the solvent materials in the first instance. These factors, in addition to the fact that certain coating characteristics may not be obtainable with solvent based liquid coating materials have led to an increasingly wide-spread utilization of electrostatic powder coating techniques. In powder coating processes, dry thermoplastic powder is entrained in a directed flow of carrier gas, and the suspended and entrained particles of powder are given a high voltage electrostatic charge much in the same manner as in liquid electrostatic processes. The charged dry particles are thus caused to be attracted to the workpieces to be coated. After application of the coating, the workpiece is exposed to heat to fuse the deposited thermoplastic particles, to thereby achieve the desired homogeneous coating.
As will be appreciated, the application of powder coating materials requires careful control over the flow of powder, so that the powder is properly deposited or permitted to deposit upon the work in the first instance, and is not subsequently blown off of the workpiece by uncontrolled air flows, prior to fusing of the powders. In addition, care must be exercised in and about the spray booth area, so that excess coating material, usually referred to as overspray, does not escape into the general atmosphere of the plant. Thus, it is typical practice to conduct the electrostatic powder coating of workpieces in an enclosed spray booth area which is maintained at a slightly negative pressure relative to the outside ambient, to provide for a general inflow of outside air and thereby minimize or preclude the escape of powder overspray.
The present invention relates to improvements in the construction and arrangement of spray booth assemblies, intended specifically for the confinement, collection and recovery of overspray in powder coating processes, which include a number of improvements and advantages in relation to spray booth structures heretofore known and available.
In accordance with one feature of the invention, a novel and improved spray booth structure is provided which includes means for guiding and directing the flow of air, carrying entrained powder overspray, toward and into exhaust duct areas, from which the air and powder overspray may be withdrawn from the spray booth and processed for recovery of the powder and purification of the air. Of course, the withdrawal of air and overspray for powder recovery is, as a generality, well known. However, the present invention includes specifically novel arrangements for this purpose which provide for controllable and optimum flows of air within the spray booth enclosure, in about the vicinity of the workpiece being coated, such that air velocity in the vicinity of the workpiece is maintained both low and relatively uniform. In this connection, the spray booth structure of the invention includes, among other things, means for encouraging the development of a transverse air velocity gradient within the spray booth area, providing for increased downward air velocity in the vicinity of the spray booth walls and relatively reduced downward air velocity in the immediate vicinity of the workpiece. This has the twofold advantage of minimizing air disturbance around the workpiece, which might otherwise interfere with proper deposition and/or retention of the powder coating material, while at the same time providing for a desirably greater flow velocity along the spray booth walls to minimize the deposition of powder on such walls.
The spray booth structure of the invention is useful to great advantage in connection with conveyor supported workpieces, which are carried through the spray booth area and, while confined within the booth, are exposed to the application of electrostatically charged powder spray particles, typically by means of vertically reciprocating spray devices. Usually a spray booth of the type contemplated by the invention is somewhat elongated in the direction of conveyor travel, and the spray booth structure of the invention advantageously includes overspray recovery means of an improved and optimized form arranged to provide a desired velocity gradient across the width of the booth, as referred to above, while at the same time providing for maximum uniformity of downward air velocity in the lengthwise direction. Thus, ideally, the vertically downward air velocity in the booth in a given vertical plane, extending lengthwise of the spray booth, should be uniform throughout the length of the booth. Such ideals are, of course, not capable of being fully realized. Nevertheless, the apparatus of the invention serves to optimize the air flows in the direction of the desired ideal.
In accordance with another aspect of the invention, a powder containment and recovery booth is provided which is of a highly simplified structural make up and yet which is capable of a modular assembly, such that a single basic booth skeleton may accommodate a wide variety of specific booth structures tailored to the specific needs of the customer.
A further significant feature of the invention resides in the provision of a spray booth intended especially for electrostatic powder coating, and including overspray recovery means, which in addition to having the various advantageous features mentioned above is especially configured and constructed throughout to reduce to an absolute minimum the presence of surfaces within the booth on which powder overspray might collect and be retained, thus interfering with the efficiency of the overspray recovery and complicating clean up of the booth as may be necessary from time to time for color change, for example. The new spray booth thus includes a novel and advantageous "shingle" type of construction in which, wherever practicable, the lower edges of vertical wall panels overlap on the inside of the wall panels or other structure below them, such that upwardly facing horizontal surfaces are substantially avoided. In this connection, the booth construction of the invention, includes a novel arrangement of removable and interchangeable modular wall panels, which can be quickly installed in the desired shingle fashion, such that the modular assembly concept is made fully compatible with the avoidance of powder collecting surfaces.
In accordance with another and more specific feature of the invention, the spray booth structure of the invention may incorporate a variety of special purpose, specifically advantageous wall panel modules. One such wall panel module includes a plurality of downwardly extending, inwardly overlapped louvers, which can be slightly opened, when desired, to provide a controlled, downwardly directed inflow of outside air along selected side walls of the spray booth, forming an effective barrier to prevent or minimize powder overspray particles from coming into contact with such walls. Other special wall panels include an advantageous form of sealing panel permitting the entry and vertical reciprocation of a spray gun or supporting arm for a spray gun, and providing optimum sealing effectiveness without compromising the ability of the spray device to be properly reciprocated.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description and to the accompanying drawings.